1. Field of the Invention
The present invention relates to a method of and an apparatus for binarizing an image signal based on an error diffusion process.
2. Description of the Related Art
For reproducing a gradation image on a display unit or a printer which is capable of displaying only binary representations, a supplied gradation image signal which is a multi-valued image signal is converted into a binary image signaled made up of only 0s and 1s. There is known an error diffusion process for binarizing a multi-valued image signal.
According to the error diffusion process, when a multi-valued image signal representing an input pixel is converted into a binary image signal by comparison with a threshold signal, an error produced by the binarization is distributed and added to pixels in the vicinity of the input pixel, and a resultant multi-valued image signal is binarized as a new multi-valued image signal representing the input pixel.
If it is assumed that a multi-valued image signal representing an input pixel (x,y) is represented by I(x,y) (x indicates the position of the input pixel in a main scanning direction and y indicates the position of the input pixel in an auxiliary scanning direction) and a binary image signal converted from the multi-valued image signal is represented by P(x,y), then a binarization error signal E(x,y) is expressed by: EQU E(x,y)=I(x,y)-P(x,y). (1)
The binarization error signal E(x,y) determined according to the equation (1) is diffused into nearby pixels around the pixel (x,y) under consideration, according to equation (2): EQU I(x,y)=I.sub.0 (x,y)+.DELTA.E(x,y), (2)
replacing the multi-valued image signal I(x,y) representing the input pixel. The replaced multi-valued image signal I(x,y) is compared with a predetermined threshold signal, producing a binary image signal P(x,y). In the equation (2), ##EQU1## where W(k,l) represents an error diffusion coefficient which is used to diffuse the binarization error signal E(x,y) at a certain ratio.
The above error diffusion process for converting multi-valued image signals into binary image signals is effective in reproducing continuous gradation signals representing halftone-dot images or photographic images while suppressing the generation of moire patterns.
However, the error diffusion process is disadvantageous in that it allows a striped pattern peculiar to a binarized image or an undesirable texture (fine pattern) in a range of certain density levels to be generated depending on the manner in which the error diffusion coefficient W(k,l) is established. Furthermore, a dot pattern having different densities in main and auxiliary scanning directions may be produced thereby giving raise to some periodical image patterns.
For example; if a "Floyd-type filter" is used (see FIG. 4A of the accompanying drawings where * represents a pixel under consideration) for establishing error diffusion coefficients W(k-1,1-1), W(k,1-1), W(k+1,1-1), W(k-1,1) respectively with respect to multi-valued image signals I(x-1,y-1), I(x,y-1), I(x+1,y-1), I(x-1,y) that are located within two pixels from a multi-valued image signal I(x,y) of a pixel under consideration then a dot pattern is produced which is dense in a main scanning direction and coarse in an auxiliary scanning direction in highlight and shadow ranges of a binarized halftone dot image, making the image form with noticeable regular patterns, and a distinct staggered texture appears in intermediate-density ranges.
If an "elongate rectangular filter" is used (see FIG. 4B of the accompanying drawings) for establishing an error diffusion coefficient W(k,l) with respect to a multi-valued image signal I(x,y) that is located within four pixels in a main scanning direction and two pixels in an auxiliary scanning direction from a multi-valued image signal I(x,y) of a pixel under consideration then a texture which is long in the main scanning direction is generated in intermediate-density ranges of a binarized halftone dot image, and a dot pattern is-produced which is dense in the main scanning direction and coarse in the auxiliary scanning direction in ranges of the binarized halftone dot image except for highlight ranges, intermediate-density ranges, and shadow ranges.
If a "Jarvis-type filter" is used (see FIG. 4C of the accompanying drawings) for establishing an error diffusion coefficient W(k,l) with respect to a multi-valued image signal I(x,y) that is located within three pixels from a multi-valued image signal I(x,y) of a pixel under consideration then a dot pattern is produced which is dense in a main scanning direction and coarse in an auxiliary scanning direction in highlight and shadow ranges of a binarized halftone dot image, and a peculiar striped pattern is generated in ranges of the binarized halftone dot image except for highlight ranges, intermediate-density ranges, and shadow ranges.